Down the Lunar Rabbit-hole

July 12, 2010: A whole new world came to life for Alice when she followed the White Rabbit down the hole. There was a grinning cat, a Hookah-smoking caterpillar, a Mad Hatter, and much more. It makes you wonder... what's waiting down the rabbit-hole on the Moon?

This pit in the Moon's Marius Hills is big enough to fit the White House completely inside. Credit: NASA/ LROC/ ASU [

"They could be entrances to a geologic wonderland," says Mark Robinson of Arizona State University, principal investigator for the LRO camera. "We believe the giant holes are skylights that formed when the ceilings of underground lava tubes collapsed."

Japan's Kaguya spacecraft first photographed the enormous caverns last year. Now the powerful Lunar Reconnaissance Orbiter Camera (LROC, the same camera that photographed Apollo landers and astronauts' tracks in the moondust) is giving us enticing high-resolution images of the caverns' entrances and their surroundings.

Back in the 1960s, before humans set foot on the Moon, researchers proposed the existence of a network of tunnels, relics of molten lava rivers, beneath the lunar surface. They based their theory on early orbital photographs that revealed hundreds of long, narrow channels called rilles winding across the vast lunar plains, or maria. Scientists believed these rilles to be surface evidence of below-ground tunnels through which lava flowed billions of years ago.

"It's exciting that we've now confirmed this idea," says Robinson. "The Kaguya and LROC photos prove that these caverns are skylights to lava tubes, so we know such tunnels can exist intact at least in small segments after several billion years."

These Kaguya images show the Marius Hills pit in the context of a meandering system of volcanic rilles. Because the pit is in the middle of a rille, it likely represents a collapse in the roof of a lava tube. Credit: JAXA/SELENE [more]

Lava tubes are formed when the upper layer of lava flowing from a volcano starts to cool while the lava underneath continues to flow in tubular channels. The hardened lava above insulates the molten lava below, allowing it to retain its liquid warmth and continue flowing. Lava tubes are found on Earth and can vary from a simple tube to a complex labyrinth that extends for miles.

If the tunnels leading off the skylights have stood the test of time and are still open, they could someday provide human visitors protection from incoming meteoroids and other perils.

"The tunnels offer a perfect radiation shield and a very benign thermal environment," says Robinson. "Once you get down to 2 meters under the surface of the Moon, the temperature remains fairly constant, probably around -30 to -40 degrees C."

This cavern in Mare Ingenii is almost twice the size of the one in the Marius Hills. Credit: NASA/ Goddard/ ASU [more]

That may sound cold, but it would be welcome news to explorers seeking to escape the temperature extremes of the lunar surface. At the Moon's equator, mid-day temperatures soar to 100 deg C and plunge to a frigid -150 deg C at night.

Paul Spudis of the Lunar and Planetary Institute agrees that lunar lava tubes and chambers hold potential advantages to future explorers but says, "Hold off on booking your next vacation at the Lunar Carlsbad Hilton. Many tunnels may have filled up with their own solidified lava."

However, like Alice's White Queen, who "believed as many as six impossible things before breakfast," Spudis is keeping an open mind.

"We just can't tell, with our remote instruments, what the skylights lead to. To find out for sure, we'd need to go to the Moon and do some spelunking. I've had my share of surprises in caving. Several years ago I was helping map a lava flow in Hawaii. We had a nice set of vents, sort of like these skylights. It turned out that there was a whole new cave system that was not evident from aerial photos."

Notes: In a 1957 short story called "The Menace from Earth," Robert Heinlein describes a subterranean colony, "Luna City," on the moon. In one of Luna City's huge underground caves, "The Bat's Cave," people strap on wings and fly!

Some researchers have propose using ground penetrating radar to find out whether the lunar lava tunnels are still open. This kind of radar sends radio waves into the ground to determine the structure beneath the surface. When it's used on Earth, the reflected signal depends on the moisture content of the ground. According to Spudis, it would be difficult to interpret the data from ground penetrating radar used on the Moon. Lunar soil has little moisture, so there would be no clear signature returns. He says a robotic rover could be designed to traverse the steep slopes and rough rocky surfaces and venture into the cavern depths. Going underground, however, it would lose radio communications, so it would have to be attached to a fiber optic cable tied to a lunar base on the surface.